The present invention pertains to digital communications and transmission systems and more particularly to framing information transmitted along with data used to identify various fields within the data stream.
In the past, framing patterns have been limited to a small number of bits. For example, the T-carrier systems used in telephony originally employed a framing pattern consisting of alternating ones and zeros. Later, this framing pattern was replaced by two interleaved patterns, one to identify frames and a second pattern to identify a "superframe", a larger frame consisting of 12 ordinary frames. The first pattern is a basic pattern of alternating ones and zeros. These framing bits occur in alternate framing bit positions and identify the framing bit position within a frame of 193 bits. The second framing pattern, interleaved with the first, is a pattern of (111000). This framing pattern identifies the alignment of the superframe relative to the ordinary frames.
Another modification to the framing pattern introduced extended superframe. Extended superframing is a technique wherein the basic framing pattern occurs only every fourth frame and identifies a 24 frame pattern. Since this framing pattern is only 6 bits long, a relatively high potential for false framing is created. This can occur when a particular data bit pattern corresponds to the framing bit pattern.
In a superframe transmission system, the alternating ones and zeros with a period of four frames was sometimes imitated by the sign bit of a PCM encoded 2 KHZ sine wave from certain types of data modems. If the carrier system lost framing while this 2 KHZ modem signal was being transmitted on one of the voice channels, the framing circuit could mistake the sign bit of the voice channel for the framing bit. This situation would result in misframing for all 24 channels for considerable periods of time. Similar problems would result with other framing patterns.
The above problem could be solved by use of a longer and more complex framing pattern, but this introduces several new problems when previously known techniques are employed. The first problem is that as the framing pattern length is increased, the amount of hardware needed to generate and detect the framing pattern increases correspondingly. For example, a 12 bit framing pattern would require twice as much transmission and detection hardware as a 6 bit framing pattern. Thus, longer framing patterns necessitate increased pattern generation and detection hardware.
A second problem is that as the framing pattern length is increased, the time required to transmit the entire framing pattern increases proportionally. For T-carrier systems, the framing bit position is 1 bit out of a 193 bits and occurs only eight thousand times per second. For the extended superframing situation, the framing bit occurs only two thousand times per second. Since the entire framing pattern must be received before it can be recognized, searching for a long pattern through all possible bit positions in either the original T1 carrier or the extended superframing systems would require a great amount of time if a longer pattern were used.
Another problem with short framing bit patterns arises when multiple levels of multiplexing are employed in a system. Unless a different framing pattern is used for each multiplexing level, there is a danger of a higher level framing circuit falsely locking onto a lower level framing bit pattern. Separate framing patterns for each level would require long patterns, so that the pattern at each level could be orthogonal to patterns at lower levels. In present day T-carrier multiplexers, the problem is dealt with by using different frame length for each level of multiplexing. This causes lower level patterns to slide through the higher level patterns, eliminating confusion between them. However, the situation also requires demultiplexing of all channels in the high level stream in order to recover even one data channel at the lowest level of multiplexing.